Range:
Northern and Eastern Europe: Northern France, Germany, east to Black and
Caspian Sea basin; introduced in North America

The
attractively colored bitterling is a small, rather deep-bodied fish. It
lives in densely vegetated areas and can tolerate poorly oxygenated water.
It feeds on plants and small invertebrate animals. The breeding habits
of the bitterling are most unusual. The female develops a long egg-depositing
tube that extends from her genital opening. Using this tube, she lays her
eggs inside the gill chamber of a freshwater mussel. The male, who develops
brilliant, iridescent coloration in the breeding season, sheds his sperm
by the mussel's gills so that it is inhaled by the mussel and fertilizes
the eggs. Safe from predators, the eggs develop inside the mussel for 2
or 3 weeks, and the young leave it about 2 days after hatching. The mussel
is unharmed by this invasion.